It is known to provide an optically variable device where arrays of lenticular (part-cylindrical) lenses focus on an object plane that has multiple sets of interleaved image elements. Each set of image elements belong to a distinct image, so that as a person viewing the device changes the viewing angle, different images may become visible. Optically variable devices are also known to include two-dimensional arrays of non-cylindrical lenses, in particular spherical microlenses.
In security documents and in particular flexible security documents such as banknotes, it may be desirable to minimise the thickness of the lenticular or microlens arrays. To produce these security documents, the microlenses or lenticular arrays will necessarily have a relatively small focal length and consequently, a desirable transverse dimension would be on the order of 50-65 microns or even less.
Producing microlenses of this size places constraints on the processes used to apply the image elements on the object plane. For example, gravure printing (also known as rotogravure printing) presently only produce printed line widths of approximately 35 microns or more. With this limitation of approximately 35 microns on the line width, lenses with a transverse dimension 65 microns are only wide enough to allow simple optical variable effects.
Simple optically variable effects known as a ‘flipping image’ effect have been developed for lenses with these relatively small transverse dimensions, such as the device as described in PCT/AU2011/001095, incorporated herein by reference in its entirety. A ‘flipping image’ is an image that changes between different states, for example between a positive state and a negative state, at different angles. In this document, PCT/AU2011/001095, an optically variable device is provided with an array of microlenses or lenticules that have two sets of image elements within each field of view of a microlens or lenticule of the array. In particular it has been found that cross-talk may be minimised, if the first and second images are contrast-inverted versions of each other in conjunction with an off-focus lens design to produce a two channel ‘flipping image’. However this optically variable device is limited only to images that are monochromatic.
Another optically variable effect such as a multi-channel ‘flipping’ effect has also been developed, such as the device as described in PCT/AU2011/001063, incorporated herein by reference in its entirety. Here, an array of microlenses or lenticules with transverse dimensions on the order 65 microns is placed in an off-focus position to a set of image elements. The image elements additionally may be sub-divided into a set of sub-regions, where each image element in a sub-region is phase-displaced to the image element in another sub-region. Each sub-region now contributes a further image as the viewing angle changes. A plurality of sub-regions may be provided to produce a greyscale image that may change contrast continuously as the viewing angle changes. However this multi-channel optically variable device is limited to a monochromatic or greyscale image.
This single colour limitation is due to the inability of current printing processes to register additional (interlaced) colour layers to the first (interlaced) colour layer at the desirable line widths of approximately 35 microns or less. In particular, the registration tolerance of the gravure press is currently insufficient to successfully interlace additional colour layers underneath microlenses suitable for gravure-printed lenticular images on banknotes.
In view of the difficulties above, it is desirable to produce a security element that is more resistant to counterfeiting but still can be produced by current printing processes, such as gravure printing.